Chromium-containing "bonding coating" compositions for metal substrates that are applied prior to painting typically impart corrosion resistance to the surface of the substrate. Such compositions, when applied and cured on a metal substrate, exhibit excellent adherence to the substrate and offer many further desirable characteristics. These include substrate corrosion resistance, the advantage of metal formability without significant coating loss, and retention of substrate weldability. Much of the early efforts with bonding coatings involved developing combinations of hexavalent-chromium-containing substances, often supplied by chromic acid, with a wide variety of reducing agents. For example, U.S. Pat. No. 3,382,081 discloses combinations of organic reducing agents that can include aliphatic dicarboxylic acids in the combination.
Coating characteristics including corrosion resistance can be augmented by including in the pre-paint coating composition a particulate metal, such as finely divided zinc. For example, in U.S. Pat. No. 3,671,331 there are disclosed pulverulent metal-containing chromium bonding coating compositions. These contain, in addition to pulverulent metal such as zinc, the hexavalent-chromium-containing substance plus reducing agent for such substance.
Zinc as a metallic coating has also been widely used to reduce the tendency of iron or steel surfaces to corrode. In the past, the principal methods of applying such coatings were hot-dipping, also known as galvanizing and the electroplating of a zinc layer onto the steel. Zinc has been electroplated on the steel surfaces from various plating baths, preferably from acid plating baths, for providing protection of steel surfaces for various uses. It has also been known as in the U.S. Pat. No. 4,500,610 to improve the corrosion resistance of the coating layer by topcoating the metallic zinc with a bonding coating.
Moreover, it has been shown, as in U.S. Pat. No. 3,990,920, that desirable coating characteristics for these bonding coating compositions can be improved by exercising control over the pH of the coating composition. This is done by introducing a pH adjusting agent into the composition. The agent can enhanee compositional stability.
But such compositions must perform a wide variety of functions, and perform them well. It has been found to be extremely difficult to upgrade a particular coating characteristic without downgrading some others. Or without downgrading the coating composition itself, as by retarding shelf life, since these formulations are becoming increasingly complex.
It would nevertheless be desirable to enhance features of the coating composition such as augmented protection for the base metal where a cut or nick may penetrate the coating and expose such metal. It would moreover be desirable to have the composition useful as a coating over a wide variety of substrates including not only steel surfaces but electroplated surfaces such as metallic zinc coated substrates. It would be most desirable if the coating from the composition could furthermore protect the base metal, e.g., steel substrate or the like, where the electroplated layer has been penetrated. Such wide application of the coating composition to substrate metals could furthermore be most desirably extended to include annealed steel substrate metal. However, such metal can be sensitive to applied coating compositions where elevated temperature curing is then needed.